U.S. Pat. No. 5,447,024 describes a CLC process comprising a first reduction reactor for reducing an active mass by means of a reducing gas and a second oxidation reactor allowing to restore the active mass in its oxidized state by means of an oxidation reaction with humidified air.
The active mass changing alternately from its oxidized form to its reduced form and vice-versa follows a redox cycle. It can be noted that, in general, the terms oxidation and reduction are used in connection with the oxidized and reduced state of the active mass respectively. The oxidation reactor is the reactor wherein the redox mass is oxidized and the reduction reactor is the reactor wherein the redox mass is reduced.
The gaseous effluents from the two reactors are preferably fed into the gas turbines of an electric power plant. The process described in this patent allows isolation of the carbon dioxide from the nitrogen, which thus facilitates carbon dioxide capture.
The aforementioned document used the circulating bed technology to allow continuous change of the active mass from its oxidized state to its reduced state.
Thus, in the reduction reactor, the active mass (MA) is first reduced to the state MxOy−2n−+m/2 by means of a hydrocarbon CnHm that is correlatively oxidized to CO2 and H2O according to reaction (1), or possibly to the mixture CO+H2 depending on the proportions used.CnHm+MxOynCO2+m/—2H2O+MxOy−2n−m/2  (1)
In the oxidation reactor, the active mass is restored to its oxidized state (MxOy) on contact with air according to reaction (2), prior to returning to the first reactor.MxOy−2n−+m/2+(n+m/4)O2MxOy  (2)
The same document claims as the active mass the use of the redox pair NiO/Ni, alone or associated with binder YSZ (defined by yttrium-stabilized zirconia, also referred to as yttriated zirconia).
The interest of the binder in such an application is to increase the mechanical strength of the particles, too low to be used in a circulating bed when NiO/Ni is used alone.
Yttriated zirconia being furthermore an ion conductor for the O2− ions at the operating temperatures, the reactivity of the NiO/Ni/YSZ system is improved.
Many types of binders, in addition to the aforementioned yttriated zirconia (YSZ), have been studied in the literature in order to increase the mechanical strength of the particles at a lower cost than with YSZ. Examples thereof are alumina, metal aluminate spinels, titanium dioxide, silica, zirconia, kaolin.
Document EP-1,747,813 describes redox masses comprising a redox pair or a set of redox pairs, selected from the group made up of CuO/Cu, Cu2O/Cu, NiO/Ni, Fe2O3/Fe3O4, FeO/Fe, Fe3O4/FeO, MnO2/Mn2O3, Mn2O3/Mn3O4, Mn3O4/MnO, MnO/Mn, CO3O4/CoO, CoO/Co, in combination with a ceria-zirconia type binder allowing to increase the oxygen transfer capacity of said masses.
The reactivity of the redox masses involved in the CLC application is essential: the faster the oxidation and reduction reactions, the less the inventory of the materials required for operation of a unit is large. According to the literature (T. Mattison, A. Jardnas, A. Lyngfelt, Energy & Fuels 2003, 17, 643), the CuO/Cu pair has the highest reduction and reduction rates, before the NiO/Ni pair. The authors however note that the relatively low melting temperature of copper (1083° C.) limits its potential of use in CLC at high temperature, and the major part of the studies published on redox masses for CLC concern the NiO/Ni pair, despite its high toxicity (it is classified as CMR 1) and its high cost.
The use of the Fe2O3/Fe3O4 pair is also advantageous in relation to that of the NiO/Ni pair, despite a low oxygen transfer capacity, because of its low toxicity and low cost. However, since Fe3O4 tends to be reduced to FeO, the associated oxidation and reduction rates are reduced.
As regards more particularly the use of copper in redox masses, a publication in Fuel 83 (2004) 1749 by Diego, Garcia-Labiano et al. Shows the use of copper in CLC, the copper being deposited by impregnation on a porous support (alumina, silica, titanium, zirconia or sepiolite), and consequently a significant limitation of the amount of usable copper and therefore of the oxygen transfer capacity of the active mass. This publication specifies that the solids prepared by coprecipitation or by mechanical mixing of oxides with a high CuO content cannot be used in CLC processes.
On the other hand, another publication by these authors (L. F. de Diego, P. Gayan, F. Garcia-Labiano, J. Celaya, A. Abad, J. Adanez, Energy & Fuels 2005, 19, 1850) discloses that a 10% impregnation rate of CuO on alumina allows to avoid an agglomeration of particles harmful to the operation of the fluidized bed process, but that this agglomeration phenomenon is inevitable as soon as 20% CuO are impregnated.
The particle agglomeration phenomenon, which may compromise the use of redox masses in a fluidized bed, has also been studied by P. Cho, T. Mattison, A. Lyngfedt in Fuel 83 (2004), 1215, for redox masses comprising 60% of CuO, Fe2O3, NiO or Mn3O4 and 40% alumina used as the binder. They show that particles based on iron and copper agglomerate, unlike those based on Ni and Mn.
In general terms, the reaction of a metal (M) in the oxidation state +II (MO) with alumina leads, at the operating temperatures of CLC processes, to the formation of a spinel (MAl2O4). Spinel NiAl2O4 is poorly reactive and, according to Cho et al. (Ind. Eng. Chem. Res. 2005, 44, 668), spinels CuAl2O4 and MnAl2O4 are not reactive either. In order to avoid the oxygen capacity decrease induced by the formation through reaction of MO with Al2O3, the spinel itself can be used as a binder, but this involves an additional cost as a result of the introduction of a non-reactive metal.
Surprisingly, the claimant has discovered that using in the CLC application a redox mass of a particular type can overcome the drawbacks linked with the cost, the toxicity and the agglomeration of particles compromising use in a fluidized bed, while having a high oxygen transfer capacity and improved oxidation and reduction rates.